This invention pertains to an integrated circuit of a digital filter used in the digital luminance channel of a color-television receiver to give image enhancement ("peaking").
A digital filter of this kind is shown in the journal "Fernseh- und Kinotechnik", 1981, page 319, FIG. 4. It contains a first cascade of subnetworks the first of which includes a first delay element and a first adder for the input and output signals of the first delay element. The second cascade of subnetworks contains a second delay element, a multiplier, one input signal of which is a measure of the factor determining the image enhancement, the peaking factor k, and a summer which delivers the output signal of the digital filter. The first and second inputs of the summer are connected to the output of the second delay element and to the output of the multiplier, respectively. The delay provided by each of the delay elements is an integral multiple of the period of the clock signal of the digital filter. The clock signal frequency is equal to four times the chrominance-subcarrier frequency. From the complete network structure of this digital filter, it follows that the filter weights input signals of zero frequency differently, namely depending on the peaking factor. Therefore, different amplitude characteristics achieved by varying this factor have different DC components. If, however, this digital filter is used in the digital luminance channel of a color-television set, this property must be compensated for, i.e., the above prior art arrangement requires at least one additional multiplier which compensates for the above-mentioned dependence on the peaking factor. The additional multiplier is not shown in the above-mentioned FIG. 4 of the cited reference but its presence follows from the amplitude characteristics of FIG. 5 on page 319, where these characteristics have a constant DC component.
The digital filter described in the prior art reference thus contains a total of three multipliers, so that the overall circuit is quite expensive.